Method for processing audio signals and audio processing system for applying this method

ABSTRACT

A method for processing audio signals composed audio signals (L+R) and (L−R) are derived from left (L) and right (R) audio signals. The energy content of the composed (L−R) audio signals above a predetermined frequency value is measured and compared with a predetermined threshold value. Then, when this energy content falls below said threshold value, a signal derived from and decorrelated with respect to the composed (L+R) audio signal is added to the composed (L−R) signal to obtain an improved composed (L−R) audio signal, and left (L) and right (R) audio signals are obtained back again from the composed (L+R) signal and the improved composed (L−R) audio signal.

The invention relates to a method for processing audio signals and to anaudio processing system for applying this method.

Audio signals may be transmitted electronically, for example, overInternet. The audio signals may be transmitted in a compressed form, forexample, MP3, MP3Pro, WMA or Real Audio format, for reasons of reducedneed of transmission bandwidth. The compression factor may be variable,leading to a variety of audio signal stream bit rates, for example from16 kbit/s up to 196 kbit/s and sample frequencies from 8 kHz up to 48kHz. In many cases, the decoded audio signals are not perceptuallyidentical to the source material. Typically, for lower bit rates, suchas the widely used standard 128 kbit/s, is that artifacts are becomingaudible. Lower bit rates, such as 64 kbit/s show considerable artifacts.Artifacts may occur in correlated signals (M signals) and uncorrelatedsignals (S signals). The correlated signals typically show a reducedbandwidth, for example to 10 kHz and thus loss of detail in the trebleregion (the high frequency region), while uncorrelated signals showserious irregular dropouts (loss of bits) above 1 kHz. These dropoutsare responsible for an unstable stereo-image and apparent spurioussounds in the complete (stereo) soundstage.

The purpose of the invention is to avoid these disadvantages and toprovide for a method for processing audio signals and for an audioprocessing system in which compensation for the consequences of dropoutsin the soundstage is realized.

Therefore, according to the invention, a method for processing audiosignals is proposed in which from left (L) and right (R) audio signalscomposed audio signals (L+R) and (L−R) are derived, the energy contentof the composed (L−R) audio signals above a predetermined frequencyvalue is measured, this energy content is compared with a predeterminedthreshold value, after which, when this energy content falls below saidthreshold value, a signal derived from and decorrelated with respect tothe composed (L+R) audio signal is added to the composed (L−R) signal toobtain an improved composed (L−R) audio signal, and left (L) and right(R) audio signals are obtained back again from the composed (L+R) signaland the improved composed (L−R) audio signal. This means, that part ofthe composed (L−R) signal, lost by dropouts, is compensated by part ofthe composed (L+R) signal.

As already indicated, the invention also relates to an audio processingsystem. According to the invention this audio processing system isprovided with first combination means to derive from left (L) and right(R) audio signals composed audio signals (L+R) and (L−R), detection andcomparing means to measure the energy content of the composed (L−R)audio signals above a predetermined frequency value and to compare thisenergy content with a predetermined threshold value, second combiningmeans to derive, when this energy content falls below said thresholdvalue, an improved composed (L−R) audio signal from a signal obtainedfrom and decorrelated with respect to the composed (L+R) audio signaland the composed (L−R) signal, and third combining means to obtain backagain left (L) and right (R) audio signals from the composed (L+R)signal and the improved composed (L−R) audio signal.

The invention will be apparent from and elucidated with reference to theexample as described in the following and to the accompanying drawing.In this drawing a FIGURE is depicted showing an embodiment of an audioprocessing system according to the invention.

The FIGURE shows first combination means 1 and 2 to derive from left (L)and right (R) audio signals composed audio signals (L+R) and (L−R).

The composed (L−R) audio signal is supplied to detection and comparingmeans 3 to measure the energy content of the composed (L−R) audiosignals above a predetermined frequency value and to compare this energycontent with a predetermined threshold value. To realize this, thedetection and comparing means 3 comprise a filter 4 in the form of a2^(nd) order Butterworth high pass filter with a cut-off frequency ofabout 3 kHz, energy measuring means 5 to detect the energy content ofthe filtered composed (L−R) audio signal, and a comparator 6 to indicatewhether or not the measured energy content is above said predeterminedthreshold value. The comparator 6 supplies a control signal P toswitching means 7. P=0 if the measured energy content is above thethreshold value, while P=1 if the measured energy content is above thatvalue.

The composed (L+R) audio signal is supplied to means 8 comprising adelay element 9 and band pass filter means formed by a high pass 4^(th)order Butterworth filter 10 with a cut-off frequency of about 1 kHz anda low pass 1^(st) order Butterworth filter 11 with a cut-off frequencyof about 6 kHz, to obtain a high frequency signal L_(hd)+R_(hd) which isdecorrelated with respect to the composed (L+R) input audio signal. Thishigh frequency signal L_(hd)+R_(hd) is supplied to the switching means 7and, if P=1, further supplied to second combination means 12 and thereinto the composed (L−R) audio signal. The output of the second combinationmeans 12 forms an improved composed (L−R) audio signal.

The composed (L+R) audio signal and the output signal of the secondcombination means, i.e. the composed (L−R) signal if P=0 or the improvedcomposed (L−R) audio signal if P=1, are supplied to third combinationmeans 13 and 14 to obtain left and right signals L′ and R′ back again.These signals L′ and R′ can, for example, be supplied to loudspeakers.

The operation of the audio processing system is as follows:

In case the output signal of the energy measuring means 5 is above thepredetermined threshold value, i.e. P=0, L′=2L and R′=2R.

In case the output signal of the energy measuring means 5 is below thepredetermined threshold value, and the measurements according to theinvention are not applied, then for low frequencies, these arefrequencies below 1 kHz, L′ and R′ can be described by the followingequations:L′ ₁=(L ₁ +R ₁)+(L ₁ −R ₁)=2L ₁, andR′ ₁=(L ₁ +R ₁)−(L ₁ −R ₁)=2R ₁,wherein the index 1 relates to the low frequencies (<1 kHz), while forlow high frequencies, these are frequencies above 1 kHz, L′ and R′ canbe described by the following equations:L′ _(h)=(L _(h) +R _(h))+0=(L _(h) +R _(h)), andR′ _(h)=(L _(h) +R _(h))−0=(L _(h) +R _(h)),wherein the index h relates to the high frequencies (>1 kHz), so that:L′=2L ₁+(L _(h) +R _(h)), andR′=2R ₁+(L _(h) +R _(h)).The high frequency signals are reproduced monophonically or, in otherwords, as a consequence of dropouts the stereo signal is narrower thanbefore encoding.

In case the output signal of the energy measuring means 5 is below thepredetermined threshold value and the measurements according to theinvention are applied, then for the low frequencies, L′ and R′ can bedescribed by the following equations:L′ ₁=2L, andR′ ₁=2R ₁,while for the high frequencies L′ and R′ are described by:L′ _(h)=(L _(h) +R _(h))+(L _(hd) +R _(hd)), andR′ _(h)=(L _(h) +R _(h))−(L _(hd) +R _(hd)),so that:L′=2L ₁+(L _(h) +R _(h))+(L _(hd) +R _(hd)), andR′=2R ₁+(L _(h) +R _(h))−(L _(hd) +R _(hd)).The high frequency signals are now reproduced as stereophonically or, inother words, in spite of dropouts, the stereo quality is substantiallymaintained.

The invention is not restricted to the described embodiment;modifications within the scope of the following claims are possible.Particularly, the filters can be chosen differently, while somevariation in the cut-off frequencies may be possible. Instead of a delayelement a Lauridsen decorrelator or some combfilter can be used tocreate a decorrelated signal to be supplied to the switching means 7.Furthermore, it may be noted that, when the stereo signals L′, R′ areapplied as input signals for a more complex surround sound reproduction,using, for example, a 2-to-5 decoder, the artifacts will be moreserious. The application of the present invention will then be moreimportant.

1. Method for processing audio signals in which from left and rightaudio signals composed audio signals and are derived, the energy contentof the composed audio signals above a predetermined frequency value ismeasured, this energy content is compared with a predetermined thresholdvalue, after which, when this energy content falls below said thresholdvalue, a signal derived from and decorrelated with respect to thecomposed audio signal is added to the composed signal to obtain animproved composed audio signal, and left and right audio signals areobtained back again from the composed signal and the improved composedaudio signal.
 2. Method according to claim 1, characterized in that thedecorrelated signal is obtained by delaying and filtering the composedsignal.
 3. Audio processing system with first combination means toderive from left and right audio signals composed audio signals and,detection and comparing means to measure the energy content of thecomposed audio signals above a predetermined frequency value and tocompare this energy content with a predetermined threshold value, secondcombining means to derive, when this energy content falls below saidthreshold value, an improved composed audio signal from a signalobtained from and decorrelated with respect to the composed audio signaland the composed signal, and third combining means to obtain back againleft and right audio signals from the composed signal and the improvedcomposed audio signal.
 4. Audio processing system according to claim 3,characterized in that the detection and comparing means comprise a highpass filter, energy measuring means to detect the energy content of thefiltered composed audio signal, and a comparator to indicate whether ornot the measured energy content is above said predetermined thresholdvalue.
 5. Audio processing system according to claim 4, characterized inthat the high pass filter has a cut-off frequency of about 3 kHz. 6.Audio processing system according to claim 3, characterized in thatmeans are provided comprising a delay element and band pass filter meansto derive said improved composed audio signal from the composed audiosignal.
 7. Audio processing system according to claim 6, characterizedin that the band pass filter means are formed by a high pass filter witha cut-off frequency of about 1 kHz and a low pass filter with a cut-offfrequency of about 6 kHz.